The prior art generally uses a carbon substrate and a pre-oxidized fiber-reinforced or carbon fiber-reinforced composite to prepare brake discs, which, when comparing with the brake discs used to be prepared from powder metallurgy materials, have the advantages of good mechanical properties, good heat sink strength property, excellent friction performance, good product design. A prerequisite for the production of composite brake discs is to prepare an annular fibrous preform which exhibits excellent structural performance and ease of post-matrix densifying.
The prior art discloses an annular fibrous preform prepared by superposing and needle-punching an annular layer by means of a circular needle-punching station, where the annular layer is formed by alternatively abutting two sector-shaped segments obtained by cutting from an uni-directional fabric in which the filaments distributed in the radial and tangential direction of the eventual annulus (U.S. Pat. Nos. 5,323,523, 5,705,264, 4,955,123). Less material loss is achieved by this method as comparing with the method in which cutting is performed on the intact square or circular pieces, but loss due to cutting is still considerable. The two sector-shaped parts in the method are superposed alternatively, which renders the in-plane and interlayer uniformity poor, and that when there is an odd number of the sector-shaped segment in abutting, there exists two uneven and different bonding structures of fibers within the in-plane of the preform, which damages the overall uniformity of the preform, and when there is an odd number of the sector-shaped segment in abutting, the stability of the friction material would be affected ultimately; meanwhile, the long fiber content in the tangential direction of the braking force is low and uneven, abrasion and shock resistance of the friction material after the C/C composite is formed is poor; it is also evident that, abutting is used in the sector-shaped segment of the invention, which leads to unevenness in the in-plane and interlayer, the abutting site is relatively dense and thick, which blocks the passage for the subsequent deposition of the carbon substrate at a later stage, and renders the control of the composite-forming process more difficult, and makes a significant difference in performance in the interior of the friction material, poor operating stability; further, the more the individual layers of the sector-shaped units, the thicker is the preform, the more prominent is the difference, which makes the control of the overall structure and properties more difficult.
Another commonly reported preparation method involves superposing a fibrous fabric, followed by cutting the same into annular fibrous preforms after being subjected to needle-punching (U.S. Pat. No. 5,869,411, CN patent no. 96121709, 95191073 and CN02138191.7), from which, however, the shaped annular fibrous preform thus obtained has to be subjected to removal of materials in accordance with the shape and size to be used, and thus merely 30%-55% of utilization rate of raw material can be achieved, which leads to serious wasting problem of the raw materials. Furthermore, carbon fiber employs 0°/90° fiber superposing, and thus long fiber content on the rotation surface in the tangential direction of the braking force is relatively low, which affects the uniformity and stability of the product quality.
In addition, the use of pre-oxidized fiber in the preparation of annular fibrous preform has already been disclosed in the art, which, when comparing with the mature processes (CN101575766, CN1945048, CN101503844, CN02138191.7), is superior due to its strong textile operability, which facilitates the formation of a preform that exhibits the desired characteristics by forming into short fiber felts and long fibrous fabric to be alternatively overlaid and needle-punched. However, this technique has some drawbacks. Firstly, the above-mentioned method involves also removal of material in order to obtain an annular fibrous preform, utilization rate of raw material are low; secondly, a better strength of the pre-oxidized fiber can only be achieved after tension carbonization, but with significantly poorer mechanical characteristics and greater wide spreading effect as comparing with that of the carbon fiber; further, carbonization readily leads to deformation of the pre-oxidized fibrous preform. Since pre-oxidized fiber and carbon fiber have different thermal expansion coefficient, when pre-oxidized fiber is used in combination with carbon fiber, uneven distribution of the internal stress occurs in the subsequent high temperature carbonization, which makes the control over the process more difficult and affects the functional performance of the material.
There are still rooms for improvements with respect to the above-mentioned shortcomings.